A TFT is used widely as an active element for driving an active matrix type display such as an Organic EL display or a liquid-crystal display. FIG. 1 shows one pixel PLi,j as an example of an equivalent circuit of a drive circuit of the Organic EL device (OEL) 100. Referring now to FIG. 1, the equivalent circuit includes two P-channel TFTs 101, 102 as the active elements and a capacitor (Cs) 104. The scanning line Ws is connected to a gate of the selection TFT 101, a data line Wd is connected to a source of the selection TFT 101, and a power source line Wz for supplying a constant power-source voltage Vdd is connected to a source of the drive TFT 102. A drain of the selection TFT 101 is connected to a gate of the drive TFT 102, and the capacitor 104 is formed between the gate and the source of the drive TFT 102. An anode of the OEL 100 is connected to the drain of the drive TFT 102, and a cathode thereof is connected to an earth potential (or a common potential) respectively.
When a selection pulse is applied to the scanning line Ws, the selection TFT 101 as a switch is turned on and the TFT is conducting between the source and the drain. At this time, a data voltage is supplied from the data line Wd through a section between the source and the drain of the selection TFT 101, and is accumulated in the capacitor 104. Since the data voltage accumulated in the capacitor 104 is applied between the gate and the source of the drive TFT 102, a drain current Id according to a gate-source voltage Vgs of the drive TFT 102 flows and is supplied to the OEL 100.
However, it is known that a phenomenon such that a threshold voltage Vth is shifted when a voltage is continuously applied to the gate, that is, a phenomenon called “gate stress” may occur in a case of the TFT using amorphous silicon or an organic semiconductor. For example, see a reference document S. J. Zilker, C. Detcheverry, E. Cantatore, and D. M. de Leeuw, “Bias stress in organic thin-film transistors and logic gates”, Applied Physics Letters Vol. 79(8) pp. 1124-1126, Aug. 20, 2001 (hereinafter, referred to as a non-patent document-1). This phenomenon will be described with taking the P-channel TFT as an example.
FIG. 2 shows an appearance of the shift of the threshold voltage Vth due to the gate stress. In the case of the P-channel TFT, when the gate-source voltage is set to a negative polarity (that is, Vgs<0) and is continuously applied, the Id-Vgs characteristics change in a negative direction (from a curved line 120A to a curved line 120B) as shown in FIG. 2 over time due to the gate stress, whereby the threshold voltage Vth is shifted from Vth1 to Vth2. In FIG. 2, the Vgs is assumed to be a positive value (Vgs>0) for the easiness of understanding.
In the change of characteristics of the TFT, the original threshold voltage Vth is restored by setting the gate-source voltage Vgs to 0V or a positive polarity and continuously applying the same. In contrast, when the Vgs is set to the positive polarity and applying continuously the same, the threshold voltage Vth is shifted to the positive direction over time, and when the Vgs is set to 0V or the negative polarity and applying continuously the same, the original threshold voltage Vth is restored. The shift amount increases with increase in absolute value of the gate-source voltage Vgs and the duration of application thereof. When the TFT having such characteristics is used for driving the Organic EL device, the threshold voltage Vth is shifted gradually during display.
The shift of the threshold voltage may disadvantageously result in lowering of luminance of OEL or malfunction of the TFT.
Single-crystal silicone, amorphous silicon, polycrystalline silicone or low temperature polycrystalline silicone are widely used as material for forming the TFT. Recently, a TFT formed of organic material (hereinafter, referred to as “Organic TFT”) as an active layer instead of these silicon materials attracts public attention. Low molecular or high molecular organic material in which mobility of carrier is relatively high such as Penthacene, Naphthacene, or Polythiophene system material is exemplified as materials of the organic semiconductor. Since the organic TFT of this type may be formed on a flexible film substrate, such as plastic, with a process of relatively low temperature, a mechanically flexible, light-weight, and thin-model display can be manufactured easily. The organic TFT can be formed relatively at a low cost by a printing process or a roll-to-roll process.
The above-described threshold voltage shifting phenomenon appears remarkably in particular in the amorphous silicon TFT or the Organic TFT. The shift of the threshold voltage in the Organic TFT is disclosed, for example, in the above-described non-patent document-1 (S. J. Zilker, C. Detcheverry, E. Cantatore, and D. M. de Leeuw, “Bias stress in organic thin-film transistors and logic gates”, Applied Physics Letters Vol. 79(8) pp. 1124-1126, Aug. 20, 2001).
The drive circuit for compensating the shift of the threshold voltage of the TFT and a drive method are disclosed, for example, in Japanese Laid-open Patent Application No. 2002-514320 (hereinafter, referred to as a patent document-1) and No. 2002-351401 (hereinafter, referred to as a patent document-2). The drive circuits and drive methods described in these documents are capable of regulating luminance of the light-emitting device irrespective of the shift of the threshold voltage of the drive TFT while allowing the shift of the threshold voltage of the same. However, since the drive circuits in these reference documents cannot constrain generation of the shift of the threshold voltage, increase in power consumption due to the shift of the threshold voltage cannot be prevented. When the shift of the threshold voltage of the drive TFT exceeds an allowable range, it is difficult to compensate the shift, and fluctuations in luminance or the malfunction of the TFT may be resulted. In addition, since the shift of the threshold voltage occurs in the selection TFT other than the drive TFT, when the shift of the threshold voltage in the selection TFT exceeds the allowable range, malfunction of the selection TFT may be resulted. In particular, the shift of the threshold voltage of the Organic TFT is larger than that of the low-temperature polysilicon TFT or the single-crystal silicon TFT, the fluctuations in luminance of the light-emitting device or the malfunctions of the TFT may easily be occurred in the case of the active matrix type display in which the organic TFT is used.
A configuration in which a connection of the source or the drain of the drive TFT and the capacitor with respect to the scanning line is contrived to avoid fluctuation in characteristics of the TFT (see, Japanese Laid-open Patent Application No. 2004-170815: patent document-3) and a connecting structure of the TFT for reducing the shift of the threshold voltage of an α-Si transistor (see, Japanese Laid-open Patent Application No. 2005-004174: patent document-4) are disclosed.
However, the drive circuits and methods disclosed in the above-described documents have a problem such that the configuration and operation of the circuit are complicated, and the effects thereof are limited.